In semiconductor devices, including high voltage devices, it is desirable to obtain a low on-resistance that is primarily determined by the drift region resistance. Typically, the drift region resistance of a transistor is lowered by increasing the doping level of the drift region. However, increasing the doping level of the drift region has the undesirable effect of reducing the breakdown voltage. The doping level of the drift region is therefore optimized to obtain the maximum on-resistance while still maintaining a sufficiently high breakdown voltage. As the requirements for breakdown voltages increase, the use of drift region doping concentrations to adjust on-resistance and breakdown voltages becomes more difficult.
In addition to breakdown voltages being affected by the doping concentration of the drift region, breakdown voltages are also affected by the electric field distribution inside and outside the device. As a result, there have been efforts in the art to control the electric field distribution by field-shaping methods and therefore control the on-resistance and breakdown voltage of transistor devices. For example, lateral floating coupled capacitor (FCC) structures have been used to control the electric fields in the drift region of a transistor and thereby improve on-resistance. These FCC structures include insulated trenches formed in the drift region of a transistor, which contain isolated electrodes and are parallel to the direction of current flow. These FCC structures improve transistor properties. For example, the drift region field-shaping provided by the FCC regions can desirably provide high breakdown voltage and low on-resistance simultaneously. However, there are problems associated with fabricating and using floating coupled capacitors to control the breakdown voltage and on-resistance including relying on fabrication methods that use highly doped polysilicon to fill trenches, which requires additional polysilicon deposition steps that make the process more expensive and reduces yields. Further, FCC structures made by filling trenches with highly doped polysilicon can result in FCC structures that have voids. These voids can be detrimental to an FCC device.
Therefore, there is need for FCC structures that are fabricated more efficiently and which have few or substantially no voids in the FCC.